In a digital data recording and/or reproducing apparatus, partial response (PR) is exploited in recording symbols in order to suppress inter-symbol interference. The PR means an equalization system in which the inter-symbol interference is positively applied to the waveform to achieve efficient transmission. Recently, this PR is frequently used in a so-called PRML (partial response maximum likelihood) signal processing system in which a pre-correlated data sequence is combined with the viterbi decoding system in which the maximum likelihood sequence is selected and reproduced. There are several types of the PR, from which the PR matched to characteristics of the transimission channel in use or to characteristics of the recording and/or reproducing system in use is selected.
Among these PR types, the response characteristics of the PR1(=PR(1, l)) and PR2(=PR(1, 2, l)) are represented as (1+D) and (1+2·D+D·D), where D is a unit delay or one-sample delay. With the PR1 and PR2, if it is attempted to effect equalization so as to satisfy the Nyquist standard, the characteristics achieved are the low range emphasizing characteristics. For example, in the frequency response of PR1, shown in FIG. 1, the replay signals are needed as from the DC components.
However, in a recording and/or reproducing apparatus, for example a magnetic recording apparatus, DC components are not transmitted, while low range loss is encountered due to use of a magnetic recording medium. For compensation, an integration equalizer is needed. In particular, compensation of the low range loss by the aforementioned integration equalization is critical in case of application to the PRML signal processing system, combined with the decoding of the ML system, especially in consideration of the importance in raising the waveform reproducibility.
Heretofore, such a circuit structure, shown in FIG. 2, has been known as a signal processing circuit, inclusive of an integration equalizer, implemented as an analog circuit. Referring to FIG. 2, a replay signal, supplied to an input terminal 101, has its low range components equalized by integration with an integrator 102 formed by an LPF (low-pass filter), and the amplitude of the integrated replay signal is adjusted by an amplifier 103. Since the high range components, resulting from processing by the integrator configuration, undergo phase distortion, the replay signal, supplied from the input terminal 101, are sent to a differentiator 104, formed by a HPF (high-pass filter), for compensating the phase of the high range components. The amplitude of the differentiated replay signal is adjusted by an amplifier 105. An output signal from the amplifier 105 is supplied to an adder 106 where it is subtracted from the replay signal sent from the integrator 102 through the amplifier 103. It is noted that the amplifiers 103, 105 adjust the signal gain into matching with the equalization standard for the integrator side frequency and for the differentiator side frequency, respectively. This type of the integration equalizer is termed a differentiation/integration equalizer. An output from the adder 106 is sent through an anti-aliasing LPF (low-pass filter) 107 to an AID converter (analog-to-digital converter) 108 and quantized. The quantized signal is PR-equalized by an adaptive equalizing filter 109 and taken out at an output terminal 110. In the circuit of FIG. 2, the portions encircled by a broken line (integrator 102, differentiator 104, amplifiers 103, 105, adder 106 and LPF 107) are formed by analog circuits.
The above-described circuit operates as follows: It is assumed that a NRZ (non-return-to-zero) signal pattern, shown in FIG. 3, has been recorded on a recording medium by a recording and/or reproducing apparatus. The replay signal from the recording and/or reproducing apparatus at this time is as shown in FIG. 4 and is supplied to the input terminal 101 of FIG. 2. The replay signal has its low range components extracted by the integrator 102 of FIG. 2, and an integrated output shown in FIG. 5 is obtained. The signal output from the integrator 102 is amplified by the amplifier 103 into meeting with the equalization standard for this frequency. On the other hand, the replay signal, supplied to the input terminal 101, has its high range components extracted by the differentiator 104, and a differentiated output shown in FIG. 6 is obtained. The signal output from the differentiator 104 is amplified by the amplifier 105 into meeting with the equalization standard for this frequency. The replay signal, equalized for each frequency, is supplied to the adder 106 of FIG. 2 where the replay signal sent through the differentiator 104 and the amplifier 105 is subtracted from the replay signal sent through the integrator 102 and the amplifier 103. The resulting replay signal is shown in FIG. 7. After rough equalization by the differentiation/integration equalizer, the replay signal is quantized by the A/D converter 108 of FIG. 2 and processed by the adaptive equalization filter 109 with equalization to higher accuracy.
If the case of the PR1 is taken as an example, the frequency response is as shown in FIG. 1. If, in a system with low range cut-off characteristics, such as a magnetic recording and/or reproducing apparatus, the replay signal is to be equalized to the corresponding equalization standard, the DC component is not reproduced. Thus, in order to provide a sufficient S/N ratio, the gain of the amplifier 103 on the output side of the integrator 102 of FIG. 2 is significant. If such analog circuit is to be implemented as an LSI, a large operating range needs to be provided, so that the driving current for the circuit is correspondingly increased to render it difficult to lower the voltage or power consumption.